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Robert Tilford, Romi Nijhawan; Forward displacement of expanding and contracting lines beyond their point of disappearance. Journal of Vision 2014;14(10):16. doi: https://doi.org/10.1167/14.10.16.
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© ARVO (1962-2015); The Authors (2016-present)
It is known that a moving object that vanishes is not perceived to overshoot its endpoint. For instance, the flash-lag effect (FLE) - where a moving object appears ahead of a collocated flash - is abolished when the moving stimulus disappears with the flash. Here we show that the disappearing length of an expanding (contracting) line is perceived to be larger (smaller) than an identical flashed or continuous comparison line. In three experiments (2AFC method), dynamic vertical lines expanded or contracted to the left (right) of fixation before disappearing. On the right (left) of fixation one of the following types of static comparison line was presented: 1) flashed at termination of dynamic line, 2) continuous, offset synchronous with dynamic line, or 3) continuous, no offset. In all experiments, the size of the comparison line appeared to substantially lag the size of the dynamic line. In addition, these lags tended to be greater for contracting than for expanding lines. Results for comparison lines described in 1-3 above: 1) 91ms Expanding, 177ms Contracting; 2) 143ms Expanding, 207ms Contracting; 3) 97ms Expanding, 256ms Contracting. This is the first demonstration of dynamic stimuli with abrupt offset overshooting their endpoints, and exhibiting an effect at flash-termination. Several flash-lag accounts are grounded on the absence of an effect when both stimuli vanish together. Our results show that: a) a future trajectory is not necessary for the lag effect, and b) a flash is not necessary for forward displacement. We suggest that in nature it is inferred that disappearing objects have rapidly receded, and so their final positions are extrapolated towards an implicit vanishing point. This is supported by a larger effect in contracting motion. Expanding motion is inconsistent with disappearance and so the overshoot effect is diminished.
Meeting abstract presented at VSS 2014
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